This invention relates generally to torsional coupling devices for use in drive shafts, and more particularly to a simplified compression loaded torsional coupling device for such applications which is preloaded in compression.
In service, drive shafts may be subject to unpredictable loadings. If a sudden high loading is applied to the shaft, the shaft could be permanently damaged by the high torsional stresses placed on the shaft. Drive shafts also typically undergo oscillatory loadings, i.e., loadings which are greater or less than the mean load on the shaft. Oscillatory loadings can create undesirable stresses and high noise levels during shaft operation.
To protect a drive shaft from these unpredictable and oscillatory loadings, a torsional coupling device is inserted in the shaft to provide a resilient interface between the powered or input portion of the shaft and the driven or output portion of the shaft. These torsional coupling devices utilize an elastomeric material as an interface to absorb any unpredictable or oscillatory loadings placed on the shaft, to reduce noise levels, and to allow for slight misalignments, axial displacements, or angularities between the input and output shaft portions.
The particular coupling design with which this invention is concerned is referred to as a compression loaded torsional coupling. These couplings have one coupling member mounted on the powered shaft and another coupling member on the driven shaft. Each coupling member has a like number of rigid studs or fins projecting toward the other coupling number, with the fins of each coupling member being positioned between adjacent fins of the other coupling member. Between adjacent fins is an elastomeric cushion which may be bonded to the surface of the fin.
These compression loaded torsional couplings can sustain relatively high torsional loadings for a given coupling size because of an inherent stiffening spring effect. As torque is applied to the shaft, alternate elastomeric cushions are placed in compression. As the load increases, these cushions bulge and deform, making the coupling stiffer as the torque increases.
Also, compression loaded couplings are inherently fail safe because the interpenetrating studs or fins of the coupling members will provide continuity of the coupling, and of the drive shaft, even if the elastomer cushion fails. Therefore, in those applications where the coupling must be fail safe, compression loaded couplings are desirable.
It has been found that the fatigue life of compression loaded torsional couplings can be extended if the bonded elastomeric cushions are not subjected to both tensile and compressive strains during service. By preloading the elastomeric elements in compression a sufficient amount, these cushions will be subject only to compressive strains in service, thus prolonging fatigue life. That is, when oscillatory loads are encountered, the elastomeric cushions do not pass from a compressive to a tensile condition.
Present compression loaded torsional coupling devices, however, have unduly complex designs in order to achieve the preloading necessary to prevent the elastomeric cushion from passing through the the zero strain condition to the tensile condition. An example of such devices is shown in U.S. Pat. No. 3,902,333 to Dossier. This complexity in turn makes these devices more expensive to manufacture. Additionally, present compression loaded devices cannot be preloaded to a sufficient degree to overcome the effects of all oscillatory loads which may be encountered in service.